Ready-made reading glasses are made and sold in assorted magnifications to allow wearers to choose the proper magnification for their requirements. The magnification is a measure of the diopter or focusing power of the lenses contained within each pair of eyeglasses. All diopter lenses have undesirable aberrations and prismatic effects which are encountered if viewed through any point away from the lens optical center. Viewing through significant aberrations and prism may cause eye fatigue, eyestrain, blurred vision, headaches, loss of concentration and difficulty reading or doing prolonged “near work” with eyeglasses. For that reason, guidelines set forth by the International Organization for Standardization (ISO) call for horizontal tolerances of 0.66 prism diopters (cm/m) per pair, or less to ensure comfortable vision. ISO 16034: “Ophthalmic optics—Specifications for single-vision ready-to-wear near-vision spectacles”. Aberrations and prism increase as a result of two constraints: 1) the diopter power (higher power lenses contain more aberrations and prism), and 2) the position of the line of vision relative to the optical center of the lens (looking through the inner or outer edge of the lens induces more aberrations and prism than looking through the optical center). If the wearer is looking through the optical centers of the lenses, then the aberrations and prism are minimized.
Custom made prescription eyewear is made to ensure that unwanted aberrations and prism are minimized at the lens optical center. This is accomplished by the optical provider first measuring the user's separation between left and right eye, or pupillary distance (PD). FIG. 1. illustrates the pupils 1a and 1b and the PD 2a. Once the PD is known, then the prescription eyeglasses are custom made to match the separation between the optical centers to the user's PD.
Each lens, 3a and 3b as illustrated in FIG. 2, is mounted into an eyeglass frame held by lens mountings 6a and 6b. The lens 3a and 3b placement within the lens mounting 6a and 6b determines where the lens optical centers 4a and 4b are positioned. Ideally, the optical centers 4a and 4b will coincide with each of the user's pupils. In prescription eyeglasses, the optical provider is careful to mount the lenses 3a and 3b so that the optical centers 4a and 4b are positioned directly in front of the user's pupils. Hence, the OCD 5a matches the PD, resulting in minimal aberrations and prism as the wearer gazes straight ahead, as shown in FIG. 3.
As seen in FIG. 4, the pupils 4a and 4b may be spaced such that the PD 2b may be wider than the pre-set OCD 5a between optical centers 4a and 4b. Or, as seen in FIG. 5, the pupils 4a and 4b may be spaced such that the PD 2c may be narrower than the pre-set OCD 5a between optical centers 4a and 4b. Discrepancies between PD and OCD create unwanted prism and aberrations.
In contrast to custom made prescription eyewear, OTC, ready-made eyeglasses, have a pre-set OCD such that only one OCD is offered by sellers. As the PD is known to vary greatly in the general population, and ranges from 45 millimeters (mm) to 80 mm, a single OCD cannot cover the range of normally occurring PDs. Thus, OTC, ready-made eyeglasses comprise excessive aberrations and prism for some wearers of eyeglasses. Aberrations and prism are more pronounced if the user requires higher powered eyeglasses or has a PD significantly wider or narrower than the offered pre-set OCD. Thus, some wearers are exposed to more aberrations and prism and may experience eyestrain or other symptoms. Hence, no single OCD setting is capable of minimizing aberrations and prism for all PDs.
U.S. Pat. Nos. 6,910,769 and 4,898,459 both disclose reading spectacles comprising a film overlay. The overlay enables a person to identify a pair of spectacles having the correct optical center (i.e., OCD) to match the wearer's pupillary separation (i.e., the PD). The '769 patent discloses a combination of a pair of spectacles having two near focal powers and a type of display mechanism that allows the purchaser to choose not only the correct power needed, but also the correct pair of glasses to match the user's PD. Optical centers of finished reading glasses are marked at the place of manufacture, then a display device including an opaque polyvinyl static cling film having two holes therein is placed onto the lenses, at the place of manufacture, such that the holes in the film overlie the optical centers. When the user dons the glasses, if a solid round area is viewed, the spectacles have the correct size for the user's PD. If a figure eight or two holes is viewed, the PD is too wide. If the user views the outside edges, the PD is too narrow. By visual observation, the user can find the correctly sized reading glasses. Similarly, The spectacles disclosed in the '459 patent comprise an opaque film that allows the user to test suitability of the spectacles for their PD.
The eyeglasses comprising the film described in the '769 and '459 patents are made using a process that is expensive and interferes with vision while the user tries on the eyeglasses and attempts to look through the film. Therefore, there is a need for OTC ready made eyeglasses that do not comprise such a film and that can accommodate a variety of PDs, for single vision, bifocal and multifocal reading glasses, thus minimizing the aberrations and prism thereby increasing the comfort and wearability of the eyeglasses.